A barrel nut is a type of fastener (nut) which has a barrel-shape flange and protruding boss that is internally threaded. The boss sits within the components being fastened, and the flange provides the clamping force. The barrel nut flange and accompanying machine screw are designed to sit flush on either side of the surfaces being fastened. It is normally chosen because of its low profile compared to other nuts. The flange often has a built-in feature such as a slot to be used with a screwdriver to aid in tightening the fastener, although some barrel nuts have binding flanges that allow for one-sided assembly. Barrel nuts are also referred to as sex bolts or Chicago binding posts.